Car has to be the current car on the plate and it has to have been so licensed for at least 3 years and be either over 7yo or have over 300k kilos on the clock... but, as stated, you still get the other 10k without claiming the scrappage element... and it is €10k on top of the normal grants, if the erm got less he was done up like a kipper!
The car has to be registered before it can be licensed as a taxi hence it's a private car you're buying and registering. To qualify for the full (€10k + €10k) it must be under 3 months old with less than 3,000 kilos on the clock... so, technically, you you could buy it before applying for the grant but you'd want to be damn sure you're going to get the grant to do that, I guess.